Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2010 was about 22 million tons. About seventy percent of the ethylene oxide produced is further processed into ethylene glycol; about twenty percent of manufactured ethylene oxide is converted to other ethylene oxide derivatives and only a relatively small amount of ethylene oxide is used directly in applications such as vapor sterilization.
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Of particular interest in recent years has been the proper operating and processing parameters for the production of ethylene oxide using so-called “high selectivity catalysts”, that is Ag-based epoxidation catalysts that contain small amounts of “promoting” elements such as rhenium and cesium.
With respect to these Re-containing catalysts, there has been considerable interest in determining the optimum conditioning or start-up conditions, since Re-containing catalysts require a conditioning period to maximize selectivity.
These conditioning procedures are often directed to regulating the reactor feed during conditioning, in particular by ensuring the catalyst has a performance-enhancing amount of chloride. The presence of chloride in the reactor feed plays a key role in maintaining the catalyst's selectivity—the efficiency of the partial oxidation of ethylene to ethylene oxide. This is especially the case with respect to rhenium-containing catalysts, which are very dependent on the presence of chlorides to achieve optimal performance Examples of such procedures were previously disclosed in U.S. Pat. No. 4,874,879 to Lauritzen et al. and U.S. Pat. No. 5,155,242 to Shanker et al., which disclose start-up processes in which a Re-containing catalyst is pre-chlorinated prior to the introduction of oxygen into the feed and the catalyst is allowed to “pre-soak” in the presence of the chloride at a temperature below that of the operating temperature. While some improvement in overall catalyst performance has been reported using these prior art methods, the pre-soaking and conditioning nonetheless impose a substantial delay before normal ethylene oxide production can begin after oxygen is added into the feed. This delay in production may either partially or entirely negate the benefit of increased selectivity performance of the catalyst.
With respect to other components of the reactor feed, being conditioned in an environment that has a relatively low ethylene to oxygen ratio can also affect the performance of the catalyst. Typically Re-containing catalysts make use of relatively low ratios of ethylene to oxygen in the reactor feed to achieve maximum catalyst performance. Additionally, other patents increase the carbon dioxide in the feed to allow the reaction temperature to be increased to sufficiently to initiate or condition the catalyst while also suppressing the ethylene conversion in the reactor. An example of this procedure can be seen in U.S. Pat. No. 4,766,105 to Lauritzen, where carbon dioxide concentrations are as high as 7 mol % during the start-up.
Temperature is also an important aspect of conditioning—as shown, for example, in the proposed start-up process disclosed in U.S. Pat. No. 7,102,022 to Evans et al., which discloses contacting a Re-containing catalyst bed with a feed comprising oxygen and holding the temperature of the catalyst bed above 260° C. for a period of time of up to 150 hours. A similar technique is disclosed in U.S. Pat. No. 7,485,597 to Evans et al., but in this case the catalyst bed is held above 250° C. In both cases, after conditioning is completed and a peak selectivity value obtained, the temperature is lowered until the desired level of productivity is reached.
Again, while some improvement in catalyst performance may be obtained by this prior art method, there are also inherent disadvantages to this process. In particular, while these high-temperature conditioning processes may be necessary for activating the catalyst to obtain peak selectivity, such higher temperatures can also detrimentally modify the surface characteristics of the catalyst thus, reducing the selectivity as well as the activity of the catalyst, the latter of which forces the operator to increase temperature in order to maintain the desired level of production. The detrimental effects of high temperature conditioning may be particularly pronounced in the case of highly active Re-containing catalysts.
For these reasons there is a continuing need in the art for conditioning procedures for use in olefin epoxidation that do not detrimentally affect the performance of catalysts.